The development of resistance by cancer cells to a wide variety of chemotherapeutic agents poses a major obstacle in the successful treatment of cancer. Drug resistance is observed in a broad range of cancer cell types. Many mechanisms contribute to drug resistance, including drug inactivation, extrusion of the drug by cell membrane pumps, mutations of drug targets, and failure to initiate apoptosis. Prevention of apoptosis can result from a variety of conditions, including retention of the mitochondrial membrane potential and cytokine stimulation.
The search for proteins responsible for drug-resistant phenotypes has implicated the antiapoptotic molecule Bcl-2. Overexpression of Bcl-2 plays a role in the development of drug resistance in leukaemia and other apoptosis-prone tumours and, consequently, a poor prognosis in various human cancers. Bcl-2 belongs to a family of proteins, the Bcl-2 family, the members of which regulate apoptosis. The family includes both proapoptotic and antiapoptotic members. Although a precise understanding of how Bcl-2 exerts its antiapoptotic effects remains elusive, it has been found to be overexpressed in many cancers including lung, colorectal, prostate, and breast cancers as well as in leukaemia's and lymphomas.
Thus, Bcl-2 is a critical cellular factor, as increased expression levels of that protein confers resistance to apoptotic stimuli, thereby contributing to the pathogenesis and progression of cancer.
The process by which the mammalian immune system recognises and reacts to foreign or alien materials is a complex one. An important facet of the system is the T-cell response. This response requires that T cells recognise and interact with complexes of cell surface molecules referred to as human leukocyte antigens (HLA) constituting the human major histocompatibility complex (MHC), and peptides. The peptides are derived from larger molecules, which are processed by the cells, which in turn present the HLA/MHC molecule. The interaction of T cells and complexes of HLA/peptide is restricted, requiring a T cell that is specific for a particular combination of an HLA molecule and a peptide. If a specific T cell is not present, there is no T-cell response even if its partner complex is present. Similarly, there is no response if the specific complex is absent, but the T cell is present.
The mechanism by which T cells recognise cellular abnormalities has also been implicated in cancer. E.g. in WO92/20356, a family of genes is disclosed which are processed into peptides which, in turn, are expressed on cells surfaces, and can lead to lysis of the tumour cells by specific CTLs. These genes are referred to as the MAGE family and are said to code for “tumour rejection antigen precursors” or “TRAP” molecules, and the peptides derived therefrom are referred to as “tumour rejection antigens” or “TRAs”.
In WO 94/05304, nonapeptides are disclosed which bind to the HLA-A1 molecule. This reference discloses that, given the known specificity of particular peptides for particular HLA molecules, one should expect a particular peptide to bind one HLA molecule, but not others. This is significant, as different individuals possess different HLA phenotypes. As a result, while identification of a particular peptide as being a partner for a specific HLA molecule has diagnostic and therapeutic ramifications, these are only relevant for individuals with that particular HLA phenotype.
Thus, it is well established that peptide epitopes derived from tumour associated antigens (TAAs) can be recognised as antigens by cytotoxic T lymphocytes (CTLs) in the context of MHC molecules. However, although it is generally accepted that most if not all, tumours are antigenic, only a few are indeed immunogenic in the sense that tumour progression is readily controlled by the immune system.
To overcome this limitation, several immunotherapeutic studies have been initiated, e.g. vaccinations with TAA-derived peptides. For melanoma, the tumour for which the largest number of CTL-defined TAAs has been characterised, powerful CTL responses against antigens have been induced by vaccination and some patients experienced a complete remission of their disease. However, most of the peptide epitopes used in these vaccination trials are melanocyte specific, and these peptides cannot be applied for tumours of non-melanocyte origin. Furthermore, expression of these TAAs is heterogeneous among tumours from different patients and can even vary among metastases obtained from one patient. However, during the last couple of years a number of tumour specific peptide antigens, which are expressed in a number of different cancers, have been identified, i.e. HER-2, Muc-1 and telomerase.
Apoptosis is a genetic program of cellular suicide, and inhibition of apoptosis has been suggested to be an important mechanism involved in cancer formation by extending the life span of cells favouring the accumulation of transforming mutations. Survivin is a recently identified member of the family of inhibitors of apoptosis proteins (IAPs). In a global gene expression analysis of about 4 million transcripts, survivin was identified as one of the top genes invariably up-regulated in many types of cancer but not in normal tissue. Solid malignancies overexpressing survivin include lung, colon, breast, pancreas, and prostate cancer as well as haematopoietic malignancies. Additionally, series of melanoma and non-melanoma skin cancers have been reported to be invariably survivin positive. The overexpression of survivin in most human cancers suggests a general role of apoptosis inhibition in tumour progression, a notion substantiated by the observation that in the case of colorectal and bladder cancer, as well as neuroblastoma, expression of survivin was associated with an unfavourable prognosis. In contrast, survivin is undetectable in normal adult tissues. These characteristics qualify survivin as a suitable TAA for both diagnostic and therapeutic purposes.
Thus, during the last decade a large number of TAAs have been identified which are recognised by CTLs in a major histocompatibility complex (MHC)-restricted fashion. As survivin is overexpressed in most human cancers and inhibition of its function results in increased apoptosis, this protein may serve as a target for therapeutic CTL responses.
The survivin protein and the potential diagnostic and therapeutic use hereof are disclosed in (1) and U.S. Pat. No. 6,245,523, which are incorporated herein by reference. Survivin is a 16.5 kDa cytoplasmic protein containing a single BIR and a highly charged carboxy-terminal coiled coil region instead of a RING finger, which inhibits apoptosis induced by growth factor (IL-3) withdrawal when transferred in B cell precursors. The gene coding for survivin is nearly identical to the sequence of Effector Cell Protease Receptor-1 (EPR-1), but oriented in the opposite direction, thus suggesting the existence of two separate genes duplicated in a head-to-head configuration. Accordingly, survivin can be described as an antisense EPR-1 product. Functionally, inhibition of survivin expression by up-regulating its natural antisense EPR-1 transcript results in massive apoptosis and decreased cell growth.
U.S. Pat. No. 6,245,523 discloses the isolation of purified survivin and it provides nucleic acid molecules that encode the survivin protein, and antibodies and other molecules that bind to survivin. U.S. Pat. No. 6,245,523 also discloses anti-apoptotically active fragments of the survivin protein and variants hereof wherein an amino acid residue has been inserted N- or C-terminal to, or within, the disclosed survivin sequence. It is specifically disclosed that such peptides should contain key functional residues required for apoptosis, i.e. Trp at position 67, Pro at position 73 and Cys at position 84.
During the past decade numerous clinical trials have shown the feasibility of peptide specific vaccination to induce anti-tumor T-cell responses in cancer patients. The clinical course of the patients, however, was in most cases not improved. This discrepancy has in numerous cases been explained by immune escape mechanisms of the tumour cells. For therapeutic strategies targeting antigens that play an insignificant role in cancer growth, the selection of antigen deficient cancer cells is a well-recognised limitation.
In the case of breast cancer patients, however, a paradoxical role of Bcl-2 protein has been observed. In primary breast tumours Bcl-2 negativity has been associated with a worse clinical outcome. Additionally, it has been reported that overexpression of Bcl-2 protein is correlated with oestrogen receptor-positive tumours mediated by oestrogen receptor response elements in the promoter region of the Bcl-2 gene. The prognosis of oestrogen-positive tumours is more favourable than that of oestrogen receptor-negative tumours. Several possible explanations for these seemingly paradoxical results have been suggested, e.g. inhibitory effects of Bcl-2 on cell proliferation, regulation of Bcl-2 expression by oestrogen, and/or the presence of Bcl-2 antagonists that inhibit its cytoprotective function.
Still, the above studies also showed that overexpression of Bcl-2 in breast cancer is correlated with drug resistance, and that downregulation of Bcl-2 by antisense oligonucleotides modulates drug sensitivity in association with apoptosis. Furthermore, gene transfection of Bcl-2 into breast cancer cell lines has uniformly resulted in enhanced resistance to apoptosis. In addition, it has been described that the presence of another inhibitor of apoptosis, the protein survivin in breast carcinoma was strongly associated with expression of Bcl-2 and with reduced apoptotic index (AI) and poor overall survival. A similar association between survivin and Bcl-2 has been described in neuroblastoma, gastric cancer, colorectal cancer, and high-grade non-Hodgkin's lymphoma. Thus, in breast carcinoma as in most other human cancers, inhibition of apoptosis is a general feature, and expression of anti-apoptosis genes, e.g. survivin and/or Bcl-2 genes, may cause more pronounced antiapoptotic effects, as reflected in reduced apoptotic index. Recently, is has been shown that survivin is a target for spontaneous T-cell reactivity in patients with various cancers. These initial findings have later been confirmed and strengthened (by ourselves and others).